Why are they so Rowdy?
It's War!
In times of war, pretty much every able-bodied man is pressed into service. They might not get much in the way of equipment: armour is limited, and weapons consist of whatever's cheap, but all the same some degree of military training is given, and when the village is perceived to be in danger, all that training comes back. So what happens if the villagers see a gang of heavily armed and armoured people come in unannounced? Unless they're given a good reason to do so, they're hardly likely to welcome the adventurers with open arms, are they?
A lot of peasants (serfs or freemen) are required by law to give military service in addition to their taxes and tithes. Even if there isn't a war on, a certain amount of drilling is necessary. This may even be going on when the heroes arrive at the village. Even if the peasants aren't particularly trigger-happy, characters can usually be relied upon to rush into battle at the sight of a platoon of armed men.
Let them Eat Cake
Like exorbitant taxes, a bad harvest can threaten the peasants with starvation, especially if the lord insists on taking his usual cut regardless of how much the crops actually yield. The lack of food in the commoners' bellies can be enough to move them to violence, but usually they need a scapegoat.
A WITCH!!!!
Why do crops fail? The success of the crops and the very turn of the seasons is God's will, surely, but why would God punish his people? Because they're not being pious or righteous enough, of course! Or perhaps someone schemes to turn people from the straight and narrow and work evil magic. Everyone knows that witches make the crops fail. Naturally anyone is a likely candidate for suspicion, but old women, preferably with no family or friends, tend to draw the brunt of the peasants' anger.
Of course, if you're playing a fantasy role-playing game rather than a strict historical setting, it is entirely possible that a witch is at work.
Tradition
What if the peasants worship a more primal, savage god, though? Perhaps the bargain they have with that god involves offering him or her a sacrifice every now and again to make the land fertile once again. Now, usually they're content to do this to one of their own; they may even have a system of drawing lots or accepting volunteers for this task. But if an outsider such as one of the adventurers should come along at the opportune time, then all the better! He can go in the wickerman instead!
Power to the People!
Assuming conditions are right, peasants can wield a hell of a lot of influence. If there is a greater demand for labour, then all of a sudden peasants and their work become more valuable, and a canny freeman will know how to turn that to his advantage.